(1) Field of the Invention
The present invention relates to an integrated circuit and, more particularly, to a detection circuit for use in an integrated circuit with an oscillation circuit, which detects oscillation stoppage of the oscillation circuit.
(2) Description of the Related Art
An integrated circuit having a timer or clock function makes use of a crystal oscillator because of a high stability in its oscillation frequency characteristics. FIG. 1 shows a circuit configuration of such crystal oscillation circuit used in an integrated circuit. As shown therein, normally there are formed, on a chip of the integrated circuit, one inverter INV operating as an amplifier and a feedback resistor Rf for setting an operating point and, externally thereof, are connected a crystal resonator XL and two capacitors Cg and Cd.
The capacitance value of each of the externally connected capacitors Cg, Cd is in the order of 10.about.30 pF. and the driving capability of the amplifying inverter INV is lowered with a decrease in the power supply voltage, which sometimes results in the stoppage of the oscillating operation.
Where an integrated circuit having such a timer or clock function together with a microcomputer is incorporated in a system, if the crystal oscillation circuit stops its oscillation caused by a reason such as a drop in the power supply voltage during operation, the integrated circuit with such a timer function issues an oscillation stoppage signal and warns the microcomputer that the timer data are invalid and this is very desirable for ensuring the reliability of the entire system.
In a conventional circuit, a frequency-voltage converting circuit and a level detection circuit have been used in order to detect the stoppage of an oscillation circuit. FIG. 2 shows such a conventional oscillation stoppage detection circuit 5 in a block diagram. In this circuit, while the oscillation continues at the oscillation circuit 1, the frequency-voltage converting circuit 6 outputs an output of a constant value. When the oscillation stops, the output of the frequency-voltage converting circuit 6 becomes 0 V. This change is detected by the level detection circuit 4, whereby the stoppage of the oscillation is detected and the detection signal is outputted from a detection output terminal DT.
FIG. 3 shows in a circuit diagram some more details of the frequency-voltage converting circuit 6 of the conventional oscillation stoppage detection circuit 5 shown in FIG. 2 with blocks. The frequency-voltage converting circuit 6 is constituted by a one-shot multivibrator 61, a constant-current source 62, a switch 63 and a smoothing circuit 64.
Here, when a signal V.sub.IN of frequency f.sub.in (Hz) is inputted to the frequency-voltage converting circuit 6 from the oscillation circuit 1, the one-shot multivibrator 61 produces pulses having a pulse width of t.sub.o in synchronization with a leading edge of the input signal. This pulse signal causes the switch 63 to turn ON and OFF, and thus electric charges in proportion to the frequencies are supplied to the smoothing circuit 64. The output voltage V.sub.OUT of the frequency-voltage converting circuit 6 is assumed that the frequencies of the input signal V.sub.IN is f.sub.in, the current value of the constant-current source 62 is I.sub.o and the resistance value of the smoothing circuit 64 is R1. Then, it follows that: ##EQU1## From this equation, it can be appreciated that the output voltage V.sub.OUT of the frequency-voltage converting circuit 6 is proportional to the frequencies of the input signal V.sub.IN.
It is by appropriately setting this proportional coefficient (I.sub.o to R1) and the detection voltage of the level detection circuit 4 that the oscillation stoppage can be detected.
However, since the above conventional oscillation stoppage detection circuit 5 makes use of the frequency-voltage converting circuit 6 as explained above, it is necessary to cause the direct current to flow constantly and this results in an increase in the consumption of power.
Also, since the one-shot multivibrator 61 is used, this results in an increase in the number of elements involved and in requiring a larger chip area occupied thereby.